An optical solar reflector (OSR) is a second-surface reflector on a transparent substrate, which is attached to the outer surfaces of a spacecraft for (1) reflecting solar energy incident on the spacecraft (reflection), and (2) radiating heat energy produced in the spacecraft (emission). The desired function of optical solar reflectors on satellites is to minimize thermal variations in sensitive satellite electronics caused by the electronic themselves and by solar radiation, which cause detrimental effects on phase array device operation. Thus, one of the requirements for OSR materials is to have low .varies./E ratio (.varies.: absorption coefficient; E: emissivity).
Currently, indium-tin oxide (ITO) coated glass in conjunction with a silver layer is used as the preferred OSR tile material. Unfortunately, condensation of thin organic films from non-metallic materials out gassing (e.g., sealants) reduces the optical performance of the OSR, and consequently the thermal control feature. For example, the absorption coefficient of the OSR can increase from 0.1 to 0.4 due to the formation of contaminant films, usually 10-100 nm in thickness, over a period of years. Thus, it is critical to overcome this contamination problem in order to maintain the long term effectiveness of the OSR. The invention described herein involves the application of a thin semiconductor coating (e.g., TiO.sub.2) to promote decomposition of organic contaminants based on photocatalytic principles.
It is known that photo assisted heterogeneous catalysis can destroy hydrocarbon and chlorocarbon contaminants in water. See, for example, the following references: D. F. Ollis, "Contaminant degradation in water", Environ. Sci. Technol., 1985, Vol. 19, No. 6, pp 480-484; C. S. Turchi and D. F. Ollis, "Mixed Reactant Photocatalysis: Intermediates and Mutual Rate Inhibition", Journal of Catalysis, 1989, 199, pp 483-496; A. E. Hussain and N. Serpone, "Kinetic Studies in Heterogeneous Photocatalysis. 1. Photocatalytic Degradation of Chlorinated Phenols in Aerated Aqueous Solutions over TiO.sub.2 Supported on a Glass Matrix", J. Phys. Chem., 1988, 92, pp 5726-5731; and M. A. Fox and C. C. Chen, "Mechanistic Features of the Semiconductor Photocatalyzed Olefin-to-Carbonyl Oxidative Cleavage", J. Am. Chem. Soc., 1981, 103, pp 6757-6759. This process is achieved by illuminating a semiconductor catalyst, typically titanium dioxide (TiO.sub.2), with greater-than-bandgap ultraviolet or near ultraviolet light in order to create electron excitation within the solid. Electron-hole pairs generated by the photoexcitation can then react with water or oxygen to lead to the formation of hydroxyl and other oxygen-containing free radicals. These radicals may attack and oxidize organics.
For hydrocarbon compounds, the primary decomposition products are often carbon dioxide and water. One of the advantages of TiO.sub.2 as a photocatalyst is its chemical stability and strong oxidizing/reducing power. In fact, TiO.sub.2 has been extensively used for organic decomposition in aqueous systems based on photocatalytic principles (see the four references noted above). TiO.sub.2 has also been used for heavy metal removal in aqueous systems based on photocatalytic principles. See, for example, H. Yoneyama et al., "Heterogeneous photocatalytic reduction of dichromate on n-type semiconductor catalysts", Nature, 1979, Vol. 282, pp 817-818; and W. Lin et al., "Hexavalent Chromium at Titanium Dioxide in Aqueous Basic Media", J. Electrochem. Soc., September 1993, Vol. 140, No. 9, pp 2477-2482. In addition, TiO.sub.2 has been used for bacteria killing in aqueous systems based on photocatalytic principles. See, for example, T. Matsunaga et al., "continuous-Sterilization System That Uses Photosemiconductor Powders", Appl. Environ. Microbiol., 1988, Vol. 54, pp 1330-1333.
To our knowledge, there are no applications which teach or suggest the use of photoactive semiconductor materials like TiO.sub.2 to decompose organic residues under vacuum conditions, for example Space, where there is little or no oxygen. The invention described herein is directed to photocatalytic coatings for preventing formation of and/or decomposing organic residue formed at the OSR surface under space conditions.